(Assuming for a second that a mainstream WiFi device can actually handle >1GB/sec in a real-world situation... I doubt it)
Two scenarios I can think of where "higher speed" WiFi routers might play a useful role:
1) If you have multiple users wanting to download at high speed off the 1GB connection, a router capable of higher speed transfers can *probably* handle this scenario better (assuming that it has multiple aerials so it can receive and transmit in full duplex)
2) If you have multiple devices on the local network and you want to transfer data internally as fast as possible (at which point I would ask "why aren't you using ethernet?")